It's most likely because your breathing isn't relaxed, and you have a high voice that naturally goes to the nose. Try singing a scale of notes without moving your mouth and singing your lowest comfortable note in a relaxed way, as if you are sighing. I bet if you relax your breathing properly, then you won't sound as nasally.

Nasal resonance is important in singing, however in your case, this is a nasally voice due to incorrect breathing, and has nothing to do with nasal resonance.

Quote by JagerSlushy

regardless of whether or not you can sound good is a nasal voice, you shouldn't. incorrect technique can be unique but there's no resonance or boldness in a nasal voice.

Diaphragmatic breathing and vocal placement. zen of screaming has a good vocal placement exercise, the one with the pencil, but the rest of it blows. just watch that part on youtube.

Yeah I would totally agree that a naturally nasal voice tends to lack in many aspects and points to incorrect breathing. But it's also important to point out the difference between a nasal voice and nasal resonance. The upper 60% of the voice (excluding the range of the whistle register) relies on having a solid nasal resonance, and without that resonance the upper ranges are impossible to sing properly in.